The phase rule and its applications . per sq. cm. is equal to 0-968 atm. 3 Tammann, loc. cit., 1900, 2. I, 424 ; cf. Goossens, Arch. neerland>1886, 20. 449. TYPICAL SYSTEMS OF ONE COM PON EXT the melting point with the pressure changes; at —150, whenthe pressure is 1625 kilogm. per sq. cm., increase of pressure Pressure in atm £u- lrr» 1 *-« / 1> , / 0 Ice 11 & in i L% 1 /*- / Jcel&n/p Water 1 / JceT&TiiV \*s \* \SP Ice I v£ \Ck \o V ?22 Fig. 2 by 1 kilogm. per sq. cm. lowers the melting point by 0*012°.This curvature of the fusion curve we shall later (Chap. IV.)see to be an almost univ
The phase rule and its applications . per sq. cm. is equal to 0-968 atm. 3 Tammann, loc. cit., 1900, 2. I, 424 ; cf. Goossens, Arch. neerland>1886, 20. 449. TYPICAL SYSTEMS OF ONE COM PON EXT the melting point with the pressure changes; at —150, whenthe pressure is 1625 kilogm. per sq. cm., increase of pressure Pressure in atm £u- lrr» 1 *-« / 1> , / 0 Ice 11 & in i L% 1 /*- / Jcel&n/p Water 1 / JceT&TiiV \*s \* \SP Ice I v£ \Ck \o V ?22 Fig. 2 by 1 kilogm. per sq. cm. lowers the melting point by 0*012°.This curvature of the fusion curve we shall later (Chap. IV.)see to be an almost universalphenomenon. Equilibrium between Ice,Water, and Vapour. TheTriple Point.—On examiningthe vapour-pressure curves of iceand water (Fig. 3), we see thatat a temperature of about o°and under a pressure of about4/6 mm. mercury, the two curvescut. At this point liquid waterand solid ice are each in equi-librium with vapour at the samepressure. Since this is so, they must, of course, be in equilibrium. 28 THE PHASE RULE with one another, as experiment also shows. At this point,therefore, ice, water, and vapour can be in equilibrium, and asthere are three phases present, the point is called a triplepoint} The triple point, however, does not lie exactly at o° C, forthis temperature is denned as the melting point of ice underatmospheric pressure. At the triple point, however, the pressureis equal to the vapour pressure of ice and water, and thispressure, as we see from the tables on pp. 21 and 23, is verynearly 4*6 mm., or almost 1 atm. less than in the previous , we have just seen that a change of pressure of 1 to a change of the melting point of 0*007 6°; themelting point of ice, therefore, when under the pressure of itsown vapour, will be very nearly +0*0076°, and the pressure ofthe vapour will be very slightly greater than 4*579 mm., whichis the pressure at o° (p. 21). The difference is, however, slight,and may be neglected here. At the tempe
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